Adding modes through RandR is not supported yet, but you should still be able to add them through xorg.conf. The driver still provides a lot of detail about where various modes come from if you start X with the -logverbose 6 option or turn on the ModeDebug option. Check /var/log/Xorg.0.log to see where your unwanted modes are coming from, and then you should be able to use the various ModeValidation options to turn off implicit modes, such as NoXServerModes, NoVesaModes, NoPredefinedModes, and NoEdidModes.

Care to comment on > 400 Mhz pixelclock on geforce 600 series hardware?